Mud motor force absorption tools

ABSTRACT

A force absorbing tool is attached to a mud motor in a drilling string. The tool includes an outer housing portion that is secured to a drilling string, and an inner mandrel portion that is secured to a mud motor and is moveable axially and rotationally with respect to the outer housing portion between an axially compressed position and an axially extended position. Helical grooves are inscribed on the inner mandrel portion, and a plurality of guide pins are associated with the outer housing and disposed within the helical groove. A compressive spring member to urge the inner mandrel portion toward the axially extended position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to torque and shock absorption devices that are used within a drill string during drilling operations.

2. Description of the Related Art

Traditionally, drilling of wellbores has been accomplished using drill bits that are affixed to the lower end of a drill string. The drill string is rotated in the hole to cause the bit to drill. As an alternative to traditional drill strings, drill bits are sometimes run in on a string of coiled tubing, which is run off of a spool located at the surface of the well. The coiled tubing is not rotated and, therefore, a downhole mud motor is used to rotate drill bit at the lower end. Coiled tubing is less rigid than a traditional drilling string and, therefore, may be more vulnerable to damage associated with axial and torsional shocks.

During drilling, the drill string is subjected to severe axial and torsional forces that can severely wear or damage components of the drilling string. Additionally, these forces can prevent the drill bit from maintaining good contact with the bottom of the borehole, thereby reducing the effectiveness of the drilling operation. Axial and torsional shock forces can significantly reduce the rate of penetration for a drilling tool.

A number of shock absorbing tools have been designed to absorb torsional and/or axial forces associated with drilling. However, most of these tools are primarily designed for use with rotary drilling strings. U.S. Pat. No. 6,543,556 issued to Anderson, for example, describes an torque and shock absorber for a traditional drill string wherein a mandrel is retained within a drive cylinder with a threaded or helical engagement between the two. Similar arrangements are found in U.S. Pat. Nos. 2,754,086; 4,443,206; 2,754,086, and 1,817,067.

Problems with prior art torque absorbing arrangements is that the spiral interface used with the tool is often insufficiently robust to stand up to the rigors of a drilling environment. The use of rollers or mere interfitting threads can cause the helical interface to bind up during operation. As a result, the tool will become inoperative. In an extremely undesirable situation, the mandrel may become canted or angularly slanted with respect to the outer housing due to the inadequate spiral interface. In this instance, the ability of the bit to drill is effectively destroyed, and the bit itself or other components may become damaged.

The forces produced when running a mud motor at the bottom of a drilling string versus rotating the entire drilling string without a mud motor are similar, but different in some important ways. In mud motor applications, fluid is pumped through the drilling string to the motor. The drilling string is not rotated and, therefore, torque and speed are produced at the bottom of the well, rather than at the top of the well, and is resisted by the string above.

The present invention addresses the problems of the prior art.

SUMMARY OF THE INVENTION

The invention provides devices and methods for absorbing torsional and axial forces associated with drill strings that use drill motors, or mud motors, to operate the drill bit. An exemplary force absorption tool is described that has an inner mandrel portion that is secured to the mud motor or associated component and an outer housing portion that is secured to the lower end of the coiled tubing or other drilling string. The inner mandrel portion and the outer housing portion are operably interengaged by a spiral or helical interface so that the inner mandrel portion will move axially and rotationally with respect to the outer housing portion. The force absorbing tool provides improved operation due to use of guide pins that engage helical grooves in the inner mandrel portion. The number and arrangement of pins is particularly advantageous for maintaining proper alignment of the inner mandrel portion and the outer housing portion in true alignment and helps prevent binding or canting of the inner mandrel portion with respect to the outer housing portion. Additionally, guide pins provide improved distribution of loads, in comparison to the use of rollers or roller balls. Guide pins also produce greater torque carrying capacity than rollers or roller balls, resulting in reduced wear and tear on mating parts.

A compressible spring force urges the tool to an axially extended position with the inner mandrel portion being extended outwardly from within the outer housing portion. In a currently preferred embodiment, a number of Belleville washers provide the spring force. An enlarged piston actuated by motor differential pressure is used in conjunction with the Belleville washers to bias the tool to the extended position. These combined forces allow for a shorter, lighter-weight tool. When the drill bit encounters axial or torsional shocks, these are absorbed, or at least reduced, as the tool moves to an axially compressed condition. While moving to the compressed condition, the inner mandrel portion rotates with respect to the outer housing portion due to the helical interface provided by the seating of the guide pins within helical grooves in the mandrel portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side, cross-sectional view of an exemplary wellbore being drilled with a motorized drill bit on coiled tubing.

FIG. 2 is a side, cross-sectional view of an exemplary mud motor force absorption tool constructed in accordance with the present invention in an axially extended configuration.

FIG. 3 is a side, cross-sectional view of the mud motor force absorption tool shown in FIG. 2, now in a compressed condition.

FIG. 4 is an enlarged side cross-sectional view of a portion of the tool shown in FIGS. 1 and 2.

FIG. 5 is a cross-section taken along lines 5-5 in FIG. 4.

FIG. 6 is an isometric view of an inner mandrel portion of a mud motor force absorption tool having rounded, helical-type pins located in its grooves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic illustration of an exemplary wellbore 10 that is being drilled through the earth 12 by a drilling system 14. The drilling system 14 includes coiled tubing 16 that is being unrolled from a spool 18 and disposed into the wellbore 10. An injector system 20, of a type known in the art for use with coiled tubing, is used to urge the coiled tubing 16 downwardly within the wellbore 10. A fluid pump 22 is associated with the coiled tubing 16 so as to selectively flow fluid into and through the coiled tubing 16.

The lower end of the coiled tubing 16 is secured to a mud motor force absorbing tool 24, constructed in accordance with the present invention. The mud motor force absorbing tool 24 is, in turn, secured to a mud motor 26 of a type known in the art for creating rotational motion under the impetus of fluid flowed axially through the motor 26. The mud motor 26 is secured to a drill bit 28. The mud motor 26 rotates the drill bit 28 with respect to the coiled tubing 16 in response to fluid that is pumped through the mud motor 26 by the pump 22. It is noted that the system 14 may be used with a milling tool or other cutting tool rather than merely a drill bit. Thus, the terms “drilling system,” “drill bit,” and the like, as used herein, are intended to include any downhole cutting tool, such as a mill or other cutter.

FIGS. 2 and 3 depict the mud motor force absorbing tool 24 in greater detail. The tool 24 includes a top sub 30 with threaded connection end 32 for attachment to the coiled tubing 16. It is noted that the upper end of the tool 24 might also be secured to other drill string tools (not shown) or to threaded pipe (not shown). The top sub 30 is secured to an outer housing 34. The outer housing 34 encloses a radially enlarged spring and bearing chamber 36. Below the spring and bearing chamber 36, a reduced diameter sleeve 38 is defined within the outer housing 34. An interior stop shoulder 40 separates the chamber 36 and the sleeve 38. A pin housing 42 radially surrounds the lower end of the sleeve 38 and is secured, at its lower end, to a bottom sub 44 by set screws 46 and threading 48. The bottom sub 44 includes a threaded end connection 50 for interconnection of the tool 24 with the mud motor 26.

The spring and bearing chamber 36 contains a plurality of axially compressible Belleville washers 52 that surround an interior tubular guide member 54. In a currently preferred embodiment, there are ninety Belleville washers that are stacked in an end-to-end, opposed relation, so that they are axially compressible. A thrust bearing 56 is located within the chamber 36 immediately below the washers 52. The thrust bearing 56 rests atop inner mandrel 58.

The inner mandrel 58 includes an enlarged upper piston head portion 60 that is secured by threaded connection 62 to guide member 54. An annular elastomeric fluid seal 63 radially surrounds the piston head 60. A reduced diameter shaft 64 extends downwardly from the piston head 60 to a threaded end portion 66 that is affixed to the bottom sub 44. A sleeve 68 surrounds the shaft 64 and has multiple helical grooves (indicated at 70 in FIGS. 4 and 5) inscribed thereupon. In a currently preferred embodiment, there are four such helical grooves. However, there may be more or fewer than four.

Lying radially outside of the sleeve 68 is a pin retainer sleeve 72, which is visible in FIGS. 4 and 5. The pin retainer sleeve 72 carries a plurality of guide pins 74 that are disposed to lie within the helical grooves 70. Because the pins 74 lie within the helical grooves 70, the inner mandrel 58 can move in a screw-type fashion (i.e., axial and radial movement) with respect to the outer pin housing 42 and outer housing 34. As best shown in FIGS. 4 and 5, the pins 74 preferably present a rounded, preferably hemispherical, inner contact surface 75 that materially eases movement of the pins 74 within the helical pathways 70. In a currently preferred embodiment, there are 28 such pins 74. The pins 74 are arranged in a helical pattern to match the pattern of the helical grooves 70, as illustrated in FIG. 4, and are distributed in a spaced relation about the entire circumference of the pin retainer sleeve 72, as FIG. 5 illustrates. This configuration provides for added stability of the inner mandrel 58 as it moves within the pin housing 42 and outer housing 34.

Alternatively, the pins can comprise short, helically-shaped pins that would lie within the helical grooves 70. FIG. 6 depicts an exemplary inner mandrel 58 with helically-shaped pins 74′ residing within the helical pathways 70. The pins 74′ are essentially generally cylindrical segments having axial ends 75 and a central body portion 77 that is shaped in a curvilinear fashion to conform to the contours of the helical grooves 70. The pins 74′ would be fixedly secured within complimentary-shaped openings (not shown) within a pin retainer sleeve (not shown) that is similar to the pin retainer sleeve 72 described previously. The pins 74′ each provide a rounded and elongated contact surface to match the grooves 70. Again, the pins 74′ are arranged in a helical pattern to match the pattern of the helical grooves 70, and are preferably distributed in a spaced relation about the entire circumference of the pin retainer sleeve. This configuration provides for added stability of the inner mandrel 58 as it moves within the pin housing 42 and outer housing 34.

It is noted that an axial flow passage 76 is defined within the body of the tool 24 to allow the flow of fluids from the surface down to the mud motor 26. The inner mandrel 58, bottom sub 44, guide member 54, pin housing 72, pin retainer 42, and pins 74 collectively form an inner mandrel portion 81. The top sub 30 and outer housing 34 collectively form an outer housing portion 82.

During operation, the tool 24 is normally in the axially extended position shown in FIG. 2. The Belleville washers 52 urge the inner mandrel 58 toward this extended position by biasing the thrust bearing 56 and upper piston head portion 60 axially downwardly. When the mud motor 26 approaches stall conditions, the differential pressure above the piston head 60 is much greater than that at operating conditions. The piston 60 uses this differential pressure to create a downward force. This force is combined with that provided by the Belleville washers 52 to urge the tool 24 to its axially extended position. When sudden resistance is encountered while milling or drilling, rather than stalling the motor, the inner mandrel portion 81 of the torque absorber tool 24 counter-rotates and moves axially upwardly in response to the increased reactive torque from the motor 26. As the drill bit 28 encounters axial and torsional shock forces, these forces are absorbed by the tool 24 via compression of the Belleville washers 52. In the compressed position, the upper end 78 of the pin housing 42 contacts a downward-facing exterior stop shoulder 80 on the outer housing 34. This contact limits the upward movement of the inner mandrel 58 with respect to the outer housing 34. The shape memory of the Belleville washers 52 and the differential pressure across the piston 60 will also urge the tool 24 to return to its normally extended position depicted in FIG. 2. Extension of the inner mandrel 58 is limited with respect to the outer housing 34 by contact between the enlarged piston head 60 with the stop shoulder 40. The main function of the tool 24 is to absorb shock, vibration and impact and compensate for the mud motor 26 reactive torque to reduce the tendency to stall during drilling or milling operations. Thus, the tool 24 increases overall milling/drilling efficiencies by optimizing weight-on-bit.

Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof. 

1. A force absorbing tool for attachment to a mud motor in a drilling string, comprising: an outer housing portion that is secured to a drilling string; an inner mandrel portion that is secured to a mud motor; the inner mandrel portion being moveable axially and rotationally with respect to the outer housing portion between an axially compressed position and an axially extended position; a helical groove inscribed on the inner mandrel portion; a plurality of guide pins associated with the outer housing and disposed within the helical groove; and a compressive spring member to urge the inner mandrel portion toward the axially extended position.
 2. The tool of claim 1 further comprising a thrust bearing.
 3. The tool of claim 1 wherein the guide pins each present a rounded inner contact surface contacting the helical groove.
 4. The tool of claim 3 wherein the inner contact surface is hemispherical.
 5. The tool of claim 1 wherein the pins are arranged in a helical pattern to match the helical groove.
 6. The tool of claim 1 wherein the pins are helically shaped.
 7. The tool of claim 1 wherein there are multiple helical grooves.
 8. The tool of claim 1 wherein the compressive spring member comprises a Belleville washer.
 9. A drilling system comprising: a drilling string; a mud motor for operating a drill bit in response to flow of drilling fluid through the drilling string; a force absorbing tool incorporated within the drilling string above the mud motor; the force absorbing tool comprising: an outer housing portion that is secured to a drilling string; an inner mandrel portion that is secured to a mud motor; the inner mandrel portion being moveable axially and rotationally with respect to the outer housing portion between an axially compressed position and an axially extended position; a helical groove inscribed on the inner mandrel portion; a plurality of guide pins associated with the outer housing and disposed within the helical groove; and a compressive spring member to urge the inner mandrel portion toward the axially extended position.
 10. The drilling system of claim 9 wherein the drilling string comprises coiled tubing.
 11. The drilling system of claim 9 wherein the compressive spring member comprises a Belleville washer.
 12. The drilling system of claim 9 wherein the guide pins each present a rounded inner contact surface contacting the helical groove.
 13. The drilling system of claim 9 wherein the pins are arranged in a helical pattern to match the helical groove.
 14. The drilling system of claim 9 wherein there are multiple helical grooves.
 15. The drilling system of claim 9 wherein the pins are helically shaped.
 16. A method of absorbing axial and torsional forces associated with drilling operations by a coiled tubing-run drilling system, the method comprising the steps of: incorporating a force absorbing tool into the drilling system above a mud motor; operating a drill bit of the drilling system by flowing drilling fluid through the mud motor; and absorbing axial and torsional shocks to the drill bit by moving an inner mandrel portion of the force absorbing tool axially and rotationally with respect to an outer housing portion of the force absorbing tool, the inner mandrel further being axially and rotationally guided by an interface of sliding of pins within helical grooves.
 17. The method of claim 16 further comprising the step of urging the force absorbing tool to an axially extended position with a compressive spring member and piston. 